Resistance exercise devices are well known in the art. Resistance exercises normally involve the contraction of a muscle against an opposing resistive force to move a portion of the body through a range of motion. The contraction is usually repeated to include a plurality of cycles (repetitions) of motion of the body portion through the range of motion, which range is determined by the degree of muscular contraction and extension achieved during a repetition. The resistive force may be provided by gravity, as with weight training (barbells, dumbbells, pull-up and pull-down stacks of weights, etc.), by an elastic force such as springs, bungees, pneumatic or hydraulic mechanisms, flexible rods and the like, or by flywheel or pulley braking devices.
Weight lifting is an exercise in which muscles contract against a resistance through a range of motion. The resistance is normally in the form of a weighted object that the user moves through either a flexion or extension of a body portion such as the arms or legs. In weight lifting, there are a number of exercises in which the user moves a weighted barbell in order to strengthen his or her upper, lower and torso body muscles. One example of such an exercise is a bench press in which the individual initially assumes a supine position atop a support bench. The weightlifter then uses his or her arms to lift the barbell from a position just above the lifter's chest to a higher vertical position where the lifter's arms are fully extended. This exercise is normally accomplished without any sideways movement, such as abduction or adduction of the lifter's hands. This basic exercise can be modified by inclining the support bench (inclined press) or by starting with the bar substantially coplanar with the user's torso (pull-overs).
In the biomechanics of limb function, one or more joints contribute to the limb's functional motion. Each time the limb moves, motion takes place in one or more of these joints. Limb movement, such as movement of the arm, may include flexion, extension, abduction, adduction, circumduction, internal rotation, and external rotation. These movements are usually defined in relation to the body as a whole. Flexion of the bicep is an upward movement of the forearm towards the shoulder when bending at the elbow. Abduction is the movement of raising the arm laterally away from the body; adduction, the opposite of this, is then bringing the arm toward the side. Circumduction is a combination of all four of the above defined movements, so that the hand describes a circle. Internal rotation is a rotation of the arm about its long axis, so that the usual anterior (front) surface is turned inward toward the body; external rotation is the opposite of this, with the posterior (rear) surface turned inward.
All movements of limbs, for example, the arm relative to the shoulder, can be described by the terms used above. It will be appreciated by the artisan that most movements of a limb such as the arm are combinations of two or more of the above defined movements. A plurality of muscles cross each limb joint. Their function is to create motion, and thus the ability to do work with the limb. To perform a given task with precision, power, endurance, and coordination, most, if not all, of these muscles must be well conditioned.
The function of each of these limb muscles depends on its relative position to the joint axis it crosses, the motion being attempted, and any external forces acting to resist or enhance motion of the limb. During limb motion, groups of muscles interact so that a desired movement can be accomplished. The interaction of muscles may take many different forms so that a muscle serves in a number of different capacities, depending on movement. At different times a muscle may function as a prime mover, antagonist, or a fixator, or synergistically as a helper, a neutralizer or a stabilizer.
For example, consider flexion of the arm. There are three major joints which contribute to elbow function: the ulnar-humeral, radio-humeral, and the radio-ulnar, all referencing interaction between the three main arm bones. The ulnar-humeral is responsible for flexion and extension while the radio-humeral and the radio-ulnar joints are responsible for supination and pronation. Flexion is movement in the anterior direction from the position of straight elbow, zero degrees to a fully bent position such as a curl. Extension is movement in a posterior direction from the fully bent position to the position of a straight elbow.
A plurality of muscles effect motion at each limb joint. For example, in the elbow, these include the Biceps brachii, the Brachialis and the Triceps brachii. These muscles are continually active as their role changes in performing the complex activities of daily living. Each muscle spanning a limb joint has a unique function depending on the motion being attempted. It is generally conceded that in order to fully train and strengthen limb musculature, it is necessary to work the limb in all planes and extremes of motion to optimize neuromuscular balance and coordination.
There are three types of muscular contractions—concentric, static and eccentric. A concentric (or positive) contraction is one in which a muscle shortens against a resistance such as when you raise a weight. A static (or isometric) contraction occurs when a muscle exerts tension but there is no significant change in its length. This happens when you push or pull against an immovable object. Lastly, an eccentric (or negative) contraction is one in which a muscle lengthens against a resistance such as when you lower a weight.
The types of limb exercise and/or exercise devices currently used in exercise programs generally include isometric, isotonic and isokinetic exercise. Isometrics is an exercise that is performed without any joint motion taking place. For example, pressing a hand against an immovable object such as a wall. When exercising a muscle group within a limb, strength can be improved only in the range of motion in which the limb is being exercised. Since in isometric exercises only one position and one angle can be used at one time, isometric exercise is time consuming if done correctly.
Isotonic exercises are done against a movable resisting force. The resisting force is usually free weights. Isotonic exercises are probably the most common method for exercising when using both the upper and lower limbs as free weights are relatively inexpensive to acquire and readily available in gyms. A weight is held in the hand and moved in opposition to gravity. It is a functional advantage to be able to move a limb through a full range of motion, but because of the unidirectional nature of gravity, the body position must be continually changed for all muscles to be exercised.
During a single repetition of isotonic weightlifting, the load remains constant but the amount of stress on the muscle varies. The most difficult point in the range is the initial few degrees with a movement to overcome inertia. As the upper extremity comes closer to the vertical position, work becomes easier due to improved leverage. This creates a non-cyclic variability in the degree of muscle tension throughout the range of motion. Isotonic exercises can be performed on Nautilus and similar machines which achieve a more uniform resistance. Nautilus-type machines feature a cam-shaped pulley (shaped like the circumference of a Nautilus swirling sea shell) that provides a transmission to increase or decrease the tensile load in a cable fixed to the pulley so that the exerciser experiences a more uniform resistance. The varying tensile load adjusts to the body's natural strength curve throughout the entire range of motion, making the movement feel easier in positions where the body is weaker and more difficult where the body is stronger. For example, performing an arm curl with a free weight is more difficult at the beginning than toward the end of the motion because of increased leverage at the elbow as the curl progresses. In contrast, the cam pulley or track line of a Nautilus machine varies the resistance levels so that the effort required to begin an arm curl is approximately equal to the effort required at the end. A major disadvantage is that motion on these weightlifting machines is confined to a straight plane movement without deviation which does not replicate normal in-use movement of the limb.
Isokinetic exercise involves a constant speed and a variable resistance. The resistance imparted by these devices increases in response to increases in the force produced by the muscles, thereby limiting the velocity of movement to roughly isokinetic conditions over part of their range. The operating principle is that strength is best developed if muscle tension is kept at a maximum at every point throughout the range, though this principle has not been universally accepted. Isokinetic exercise machines are limited to movement of a limb in one straight plane, though the resistive force can be bi-directional within that plane of movement, for example, on the flexion and extension strokes of an arm curl. Each of the systems available has its own features but basically they are all the same in that they have a rotating lever arm which moves in a single plane. Moreover, the machines are typically quite expensive as they utilize servo motors and microprocessors in so-called active dynamometry. Typically, electronic servomotors or a hydraulic valve controls the lever arm in both directions. Exemplary systems are sold by Cybex, Biodex, Isocom, and Kin-Com AP.
The particular muscle fibers involved in a contraction during a single repetition of resistive exercise depends upon the direction of the resistive force vector. If the resistive force vector is constant during a repetition, both directionally and in magnitude, as is the case with most prior art resistance exercise devices, only the muscles and portions of the muscle fibers within a muscle that are necessary to counter the resistive force will contract. Pull-down/press-down (“PD2”) types of exercise devices, such as, for example, disclosed in U.S. patent application Publication No. US2002/0068666 by Bruccoleri, have been further improved to include flexible members (e.g., ropes) attached to a horizontal resistance bar. The flexible members are adapted to be grasped by the hands. In operation, the user naturally changes the direction of the resistive force vector during a repetition such that different muscles and different muscle fibers within a muscle are exercised during the repetition. The prior art pull-down/press-down resistance type of exercise devices, such as the device shown in FIG. 1, enable the user to exercise a plurality of muscles during a repetition because the user varies the plane of motion of the limbs during a repetition.
Despite many configurations of exercise machines developed over the years, there remains a need for a more holistic and effective training machine that activates a broader range of muscle groups in a single repetition.